1. Field of the Invention
The present invention relates generally to wideband power amplification, and in particular, to a predistorter and predistorting method for linearizing the non-linear distortion characteristic of a complex modulated baseband signal, caused by a power amplifier.
2. Description of the Related Art
In a typical mobile communication system that communicates via radio frequency (RF) signals, RF amplifiers are categorized as being low-power, low-noise receive amplifiers or high-power transmit amplifiers (HPA). The efficiency of the high-power transmit amplifier is a greater consideration than noise. The high-power amplifier is widely used in mobile communication applications and operates near a non-linear operation point to achieve high efficiency.
Intermodulation distortion (IMD) from the amplifier output adversely affects out of band frequencies as well as in band frequencies with spurious signals. A feed forward method is usually adopted to eliminate the spurious component. Despite the advantage of perfect elimination of the spurious component, however, the feed forward method has low amplification efficiency and requires control at an RF stage. Therefore, the HPA becomes bulky and increases system cost.
Digital predistortion (DPD) is being studied as a means of providing high efficiency and low cost in the mobile communication industry. The DPD precompensates an input signal with an inverse of the non-linearity of a power amplifier at a digital stage and renders the amplifier output linear. The non-linearity of the power amplifier shows up as Amplitude Modulation to Amplitude Modulation (AM to AM) conversion distortion and Amplitude Modulation to Phase Modulation (AM to PM) conversion distortion. The AM to AM conversion distortion is defined as a change in the amplitude of an output signal compared to the amplitude of an input signal, while the AM to PM conversion distortion is defined as a change in the phase of the output signal compared to the amplitude of the input signal.
Most predistorters apply to single tone frequency signals or narrow band frequency signals. Therefore, they generally compensate for the memoryless non-linearity of a power amplifier. The memoryless non-linearity refers to the present output being influenced by the present input only. However, the memoryless non-linearity of the non-linear amplifier at a wideband frequency causes previous input signals as well as the present input signal to affect the present amplifier output, thereby substantially changing the AM to AM and AM to PM characteristics. This phenomenon is called memory effects. The non-linearity of a power amplifier varies with the frequency bandwidth of an input signal.
The increasing use of wideband frequencies in mobile communication systems has motivated research and development on the memoryless effects of non-linear amplifiers. A main technique of compensating for both the memoryless non-linearity and memory effects of a non-linear amplifier applies a simplified Volterra model. A Volterra series can be seen as a Taylor series with a memory. The Volterra series is used to accurately model a non-linear system. A Volterra model predistorter eliminates the non-linearity of a non-linear amplifier using an inverse of a Volterra series model that accurately simulates the non-linearity.
For the Volterra model, the predistortion characteristic to linearize a power amplifier with a memory is expressed as a discrete Volterra series with a finite memory. A signal d(n) predistorted by modifying the discrete Volterra series to a finite discrete Volterra series is represented byd(n)=hvolterra(n)EXvolterra(n)  (1)And a Volterra kernel vector hvolterra and an input signal vector xvolterra are given ashvolterra(n)=[h1(0),h1(1),h1(2), . . . ,h1(m−1),h3(0,0),h3(0,1),h3(0,2), . . . ,h3(0,m−1), . . . ,h3(1,0),h3(1,1),h3(1,2), . . . ,h3(1,m−1), . . . ,h3(m−1,m−1)]xvolterra(n)=[x(n),x(n−1),x(n−2), . . . ,x(n−m−1),x(n)|x(n)|2,x(n−1)|x(n)|2,x(n−2)|x(n)|2,. . . ,x(n−m−1)|x(n)|2, x(n)|x(n−1)|2,x(n−1)|x(n−1)|2, . . . ,x(n−m−1)|x(n−1)|2, . . . ,x(n−m−1)|x(n−m−1)|2]T  (2)where hi(m,n) is the complex predistortion gain for an ith-order signal, that is, the gain of mth and nth previous input signal samples in combination. As noted, this predistorter is configured in an Finite Impulse Response (FIR) structure and considers previous input signal samples up to an mth one.
The predistorter generates the predistorted signal d(n) by multiplying a complex input signal by the complex gain. After the signal is amplified in an HPA, the signal d(n) is linearized. By separating the Volterra kernel vector hvolterra and the input signal vector xvolterra into in-phase (I) signal components and quadrature-phase (Q) signal components, multiplication in the predistorter is expressed as(A+jB)(p+jq)=Ap−Bq+j(Aq+Bp)  (3)where A and B denote the I and Q signal components of an input signal, respectively, and p and q denote I and Q predistortion gains extracted by an adaptation algorithm, respectively.
As noted from Eq. (3), the predistorter using the discrete Volterra series experiences a rapid increase in computation volume with a modulation order. Moreover, if an input signal vector is formed from previous (m31 1) values and applied to the input of a power amplifier influenced by mPA finite memory samples, the number of previous input signal samples that affect the power amplifier is mPA+m−1. Thus, the power amplifier is affected by more memory samples than the predistorter and the predistorter fails to appropriately linearize the non-linearity of the power amplifier. This is attributed to lack of sufficient information required to generate a predistorted signal in the predistorter.
Despite different distortions in the I and Q signals of the amplifier output, the same predistortion gain is multiplied by the I and Q signals, thereby limiting predistortion gains. Therefore, errors may occur in the predistortion signal for linearizing the power amplifier and full linearization cannot be achieved.